1. Field of the Invention
The present invention relates to a device for correcting DC components generated by an orthogonal modulator or the like in radio communication equipment and a method thereof.
2. Description of the Related Art
Recently, in radio communication equipment, an orthogonal modulation for generating modulated signals as a product sum of two signals which are orthogonal to each other of a carrier wave signal and an input signal (I and Q channels) has become used in order to effectively utilize a frequency or so. A direct RF modulation method for directly modulating base-band signals to RF signals without passing them through an intermediate frequency has been adopted in order to miniaturize a transmitting unit.
In radio communication equipment adopting the direct RF modulation method for such orthogonal modulation, it is known that DC components (DC offset) are generated in a D/A converter or a modulator and carrier leak (local leak) is externally outputted. Since this local leak is an unwanted wave, in order to realize communication with good quality, the local leak must be reduced. In order to reduce this local leak, a DC offset correction circuit for realizing a function to give an offset voltage so as to cancel the DC offset generated in a modulator or the like is needed. Since the amount of DC offset of the modulator or the like changes depending on temperature and the amplitude of I and Q signals to be inputted, it is preferable to update the parameter of the DC offset correction circuit and adaptively cancel the DC offset even when the radio communication equipment is operated. Thus, a device for reducing local leak even when temperature and IQ amplitude values change, by a CPU provided for the radio communication equipment calculating DC components using reference signal data or feedback signal data and adaptively updating the parameter of the DC offset correction device is realized.
As the conventional DC offset correction method, two methods are known.
One method receives transmitting signals by a feedback loop in the radio communication equipment, extracts DC offset components only by this feedback signal and corrects the DC offset by a transmitting unit, based on the extracted DC offset components. The correction by this method is called feedback type DC offset correction or feedback signal integration type DC offset correction.
The other method extracts DC offset components from the difference between the feedback signal and transmitting signal and corrects them by a transmitting unit. The correction by this method is called reference signal type DC offset correction or signal comparison type DC offset correction.
FIG. 1 shows an example of the configuration of the transmitting unit of a direct RF modulation radio communication device with a DC offset correction function. FIG. 1 shows the configuration corresponding to the two correction methods. The routes of I and Q channel signals are collectively shown.
According to the configuration example shown in FIG. 1, a transmitting signal inputted to the transmitting unit is inputted to a DC offset correction unit (10) and when adopting the reference signal type DC offset correction, is also stored in a memory circuit 2 (90) via a delay circuit (91) as a reference signal (Ref signal). In the DC offset correction unit (10), a DC offset correction process is applied to the transmitting signal inputted there by the DC offset correction control of a CPU (60) based on data stored in a memory circuit 1(80) or the memory circuit 1 (80) and memory circuit 2 (90). The transmitting signal to which the DC offset correction process is applied is inputted to a DA converter (20), is converted from a digital signal to an analog signal by the DA converter (20) and is inputted to an orthogonal modulator (30). The orthogonal modulator (30) generates a modulation wave from the transmitting signal converted into an analog signal and an RF band carrier signal which is the output of a first local oscillator (51) and outputs it to a main amplifier (40). The main amplifier (40) amplifies the modulation wave. The amplified modulation wave is externally outputted, also is fed back to the transmitting unit and is inputted to one input port of a mixer (31).
To the other input port of the mixer (31), the output of a second local oscillator (52) is inputted. The output of the mixer (31) can be obtained by converting the modulation wave into a base-band signal. The base-band signal is converted from an analog signal into a digital signal by an AD converter (120). The digital signal is inputted to a demodulation unit (130) together with the output of a numerical control oscillator (70) whose phase can be adjusted by the CPU (60), is demodulated and is stored in the memory circuit 1 (80) as feedback signal data.
In the feedback type DC offset correction method, a CPU (60) consecutively reads feedback signal data stored in the memory circuit 1 (80) to integrate them, surmises the direction of the DC offset vector based on the integration value and set the parameter for giving appropriate amplitude in the inverse direction, thereby canceling DC offset.
In the reference signal type DC offset correction method, a feedback signal obtained by demodulating the output of the main amplifier (40) to base-band IQ signals via a feedback route and a reference signal which is a base-band signal before modulation are used. DC offset is removed by subtracting the reference signal from the feedback signal, calculating a phase-reversed parameter using an error signal obtained by extracting the DC offset component of the transmitting signal and updating the parameter of the DC offset correction unit (10) Prior to this operation, the phase of the numerical value control oscillator (70) must be adjusted in order to correctly calculating the error signal, the phases of the feedback signal and reference signal must be adjusted and the signal point phases of the reference and feedback signals must be matched.
The details of the above-described DC offset correction method are disclosed in the following patent reference 1.
In this case, although these DC offset corrections are made in order to correct carrier components which appear in a modulation frequency, a transmitting device sometimes transmits a CW signal (non-modulation signal) for frequency check as a part of device test.
If the same frequency as the modulation frequency is transmitted to test when this test CW signal is transmitted, in the case of the feedback type DC offset correction, the DC offset component appearing in a feedback signal and the CW signal component cannot be distinguished and DC offset correction is not satisfactorily made.
FIG. 2 explains the above-described case using a signal vector on an IQ plane.
As shown in FIG. 2, the sum of a DC offset vector which should be “0” made by correction and a CW signal vector which should be transmitted becomes a transmitting signal vector. In the case of the feedback type DC offset correction, this transmitting signal vector is attempted to correct.
In the case of the reference signal type DC offset correction, if the influence of the DC offset component is great, a correct correction value cannot be obtained. This is because a correct phase cannot be obtained since as described above, the phase of the numerical oscillator (70) in the feedback loop is matched to the phase of the DC offset component for phase adjustment.
FIGS. 3A and 3B show the above-described situation. FIG. 3A shows a case where the DC offset component is small. As shown in FIG. 3A, the CW signal vector indicated by a dotted line coincides with the feedback signal vector. Therefore, in this case, by matching the phase of the feedback signal to that of the reference signal by phase adjustment, a DC offset can be correctly calculated.
FIG. 3B shows a case where the DC offset component is large. As shown in FIG. 3B, the CW signal vector indicated by a dotted line is the difference between the feedback signal vector and the DC offset vector being the carrier component. Therefore, in this case, a wrong phase is calculated by phase adjustment and a correct DC offset cannot be calculated.
Patent reference 1: International Publication No. WO2005/025168 A1